Opt out of composited scrolling with border radius.

This currently causes correctness and memory issues due to the way the
compositor handles masking out the corners. Let's punt on this for now.

BUG=409984,365828

Review URL: https://codereview.chromium.org/544183002

git-svn-id: svn://svn.chromium.org/blink/trunk@181484 bbb929c8-8fbe-4397-9dbb-9b2b20218538